The invention relates generally to optical switches and more particularly to optical cross-connected switches having micromirrors that are individually manipulated.
Continuing innovations in the field of fiberoptic technology have contributed to the increasing number of applications of optical fibers in various technologies. With the increased utilization of optical fibers, there is a need for efficient optical devices that assist in the transmission and the switching of optical signals. At present, there is a need for optical switches that direct light signals from an input optical fiber to any one of several output optical fibers, without converting the optical signal to an electrical signal.
The coupling of optical fibers by a switch may be executed using various methods. One method of interest involves employing a micromirror that is placed in the optical path of an input fiber to reflect optical signals from the input fiber to one of alternative output fibers. The input and output fibers can be either uni-directional or bidirectional fibers. In the simplest implementation of the mirror method, the input fiber is aligned with one of two output optical fibers, such that when the mirror is not placed in the optical path between the two fibers, the aligned fibers are in a communicating state. However, when the mirror is placed between the two aligned fibers, the mirror steers (i.e., reflects) optical signals from the input fiber to a second output fiber. The positioning of the mirror relative to the path of the input fiber can be accomplished by using an apparatus that mechanically moves the mirror. There are rumber of proposals to using micromachining technology to make optical signals. In general, the proposals fall into two categories: in-plane free-space switches and in-plane guided wave switches. Free-space optical switches are limited by the expansion of optical beams as they propagate through free space. For planar approaches, the optical path length scales linearly with the number of input fibers. Switches larger than 30xc3x9730 require large mirrors and beam diameters on the order of 1 millimeter (mm). For these planar approaches, the number (N) of input fibers scales linearly with the beam waist and the size of the optical components. Thus, the overall switch size grows as N2. It is estimated that a 100xc3x97100 switch would require an area of 1 m2, which would be a very large switch. Moreover, constraints such as optical alignment, mirror size, and actuator cost are likely to limit the switch to much smaller sizes. One planar approach claims that the optical switch can be designed so that it scales with the optical path difference, rather than the overall optical path. If this is possible, it would certainly allow larger switches. However, the optical path difference also scales linearly with the number of input fibers for a planar approach, so the switch grows very large as it is scaled to large fiber counts.
For guided wave approaches, beam expansion is not a problem. However, loss at each cross point and the difficulty of fabricating large guided wave devices are likely to limit the number of input fibers in such switches.
For both approaches, constraints such as loss, optical component size, and cost tend to increase with the number of fibers. There is a need for an optical cross connect switch which scales better with the number of input and output fibers. Some free-space optical systems can achieve better scaling. These systems make use of the fact that it is possible to use optical steering in two directions to increase the optical fiber count. Recently, optical switches that use such mirrors have been announced. The systems use piezoelectric elements or magnetically or electrostatically actuated micromirrors. The actuation method for these approaches is often imprecise. To achieve a variable switch, it is typically necessary to use a very high level of optical feedback.
U.S. Pat. No. 5,621,829 to Ford and U.S. Pat. Nos. 5,524,153 and 5,177,348 to Laor describe known optical switching systems. In one embodiment of the system of Ford, two prisms are connected to scan mechanisms that rotate the prisms. The system also includes a fixed mirror. Light from input fibers is redirected by the prisms and reflected by the fixed mirror to a particular output fiber. The rotations of the prisms determine the optical coupling from the input fibers to the output fibers. In an alternative embodiment, the function of the rotatable prisms and the fixed mirror is performed by a rotatable mirror. Thus, manipulation of the rotatable mirror determines the optical coupling between input and output fibers. With regard to the Laor system, a module of input fibers may contain fixed and movable mirrors for redirecting emitted optical signals. A concern is that the physical requirements of the mirrors presents a limitation on the center-to-center spacing of the optical fibers.
What is needed is an optical switch and a switching method that accommodate a high density of fibers without requiring exacting manufacturing tolerances in order to efficiently couple an input optical conductor to any one of a number of alternative output optical conductors.
An optical switch utilizes xe2x80x9ccomposite mirrorsxe2x80x9d (i.e., support structures having individually addressable pivotable mirrors on one side and a fixed mirror arrangement on another side) in order to provide an easily scalable arrangement for efficiently coupling optical inputs to alternative optical outputs. In the preferred embodiment, separate composite mirrors are used in each reflection from an optical input to an optical output, with at least one fixed mirror and at least two pivotable mirrors defining the optical path. Manipulation of the first encountered pivotable mirror determines the target optical output, while manipulation of the second pivotable mirror provides the desired angle for achieving a low loss coupling to the target optical output.
While not critical, the optical inputs may be arranged in a number of discrete modules of input collimators, while the optical outputs may be arranged in a matching number of modules of output collimators. An optical beam which is emitted from a particular input collimator impinges a first fixed mirror that is angled so that the beam is directed toward a first micromirror that is mounted for rotation about two perpendicular axes. The first fixed mirror and the first dual axis micromirror are on separate composite mirrors. Manipulation of the first dual axis micromirror redirects the beam to a particular second dual axis micromirror that is dedicated to the target output collimator. The reflected beam from the second dual axis micromirror impinges a second fixed mirror, which redirects the beam to the target output collimator. Also in the preferred embodiment, the beam path from the first fixed mirror to the first micromirror is perpendicular to the face of the array of input collimators, while the beam path from the second micromirror to the second fixed mirror is perpendicular to the face of the array of output collimators. Thus, the fixed mirrors are positioned at 45xc2x0 angles with respect to the associated modules.
In an alternative embodiment, the optical beams from input collimators reflect from a first dual axis micromirror before striking the first fixed mirror. As compared to the first embodiment, this embodiment has the disadvantage of increasing the module spacing, since the fixed mirror must be extended in length in order to accommodate the angular range of the optical beam. However, one advantage of this embodiment is that the micromirror size is not affected by the angle of the fixed mirror. For instance, a wedge-shaped spacer can be placed between the first fixed mirror and the first dual axis micromirror. This causes the angular range of the optical beams leaving the fixed mirror to be centered around a ray that is not normal to the input collimator from which the beam was emitted. This embodiment has the further benefit that the beam paths between the array of input collimators and the array of first dual axis micromirrors is reduced relative to the first embodiment, so that the mechanical alignment of the two structures is simplified.
In a third embodiment, the possible angular range of the optical beams is consistently centered on the middle of the opposing set of modules of collimators. In this embodiment, the module design is symmetrical about the center of the optical switch. An optical beam from one input array of collimators strikes a fixed mirror and travels away from the center of the optical switch to an associated dual axis micromirror. Preferably, the optical beam strikes the surface of the first micromirror at an angle that is perpendicular to the surface that supports the array of first dual axis micromirrors. For at least some of the composite mirrors, the arrays of micromirrors are rotated with respect to the fixed mirrors. A benefit of this embodiment is that the opposing collimator modules completely fill the field of view of each micromirror. A straightforward modification of this embodiment would be to orient the composite mirrors so that the beams travel toward the switch center (as opposed to traveling away from the switch center) after striking the first fixed mirror. This design has the benefit of reducing the optical path length, but has the disadvantage of orienting the micromirror arrays obliquely to the beam paths, thereby requiring larger micromirrors.
An advantage of the invention is that the fixed mirror on one side of a composite mirror folds the beam paths, so that the micromirror arrays can be placed very close to the collimator arrays. This ensures that most of the optical path length is between the two micromirror arrays. By combining each fixed mirror with a micromirror array into a single structure, the spacing between modules can be reduced. Thus, using the composite mirrors increases the maximum number of optical inputs and optical outputs for optical beams having fixed waists.